The male duck's penis is spiral-shaped: like a corkscrew, it twists in a counter-clockwise direction so that sperm will target the oviduct on the female's left-hand side. In almost all birds only the left ovary is functional, but in a 2007 study, Brennan and colleagues noticed that in ducks the female's vagina twists in the opposite direction.

Brennan thinks that, while the males are evolving long and flexible penises to help them force copulations, the females are using their complex vaginal anatomy to take back control over which sperm fertilises their eggs. When a female wants to mate with her chosen partner, she can make the process easier by relaxing the muscles around the vagina entrance.

Brennan's team also timed the male's penis eversion, which took a mere one-third of a second - around 60 times faster than was previously thought (see video above). "This definitely gives the males a mechanism by which they can copulate," says Brennan, who was taken aback by the speed. "To be totally honest, I'm still in shock," she says.